1. Field of the Invention
The present invention relates to impact energy absorbing composite materials used in the protection of equipment or people.
2. Description of Related Art
Protecting bodies from high energy impacts has been a long-felt need in the design of many commonly used devices, ranging from the inside surfaces in automobiles, e.g., dashboards; to protective athletic gear, e.g., chest protectors and mouth guards; to shoes, e.g., heel inserts; to various bathroom fixtures, e.g., bathtubs. Further, it is often desirable in the design of mechanical equipment to protect against sudden unexpected impacts, e.g., when hand-held tools are accidentally dropped or when heavy objects fall on stationary equipment. One common solution is to affix a layer of a polymeric material, for example, a polymer foam, on or near the surface of either the body that is to be protected, or the surface that will be impacted. In the case of humans, the force felt by the body is reduced during impact, thereby reducing the risk of injury. Further, the material acts to reduce the body's acceleration and thereby its velocity in response to the impact. By so doing, these materials reduce the trauma of the impact. In the case of mechanical objects, the foam acts in a similar fashion to reduce the force and minimize the change in velocity felt by the impacting or impacted object, thus reducing or eliminating damage.
Various materials to protect people and objects from damage due to high energy impacts have been developed. These materials typically are open or closed cell foams of various thermoplastic polymers including polyurethanes, polyethylene, polystyrene, etc., as well as foams or dense bodies of elastomeric polymers, including silicones, ethylene vinyl acetate (commonly referred to as EVA), ethylene-propylene rubbers (commonly referred to as EPM), ethylene-propylene-diene rubbers (commonly referred to as EPDM), etc. The specific polymer used depends upon the details of the application, including the degree of protection required, the allowable thickness, the cost, the ability to process into the required shape, and so forth. For any given application, these factors generally narrow the candidate materials to just a few that are seen in commercial products. As just one example, some commercially available materials used for the specific application of heel inserts include Plastazote (Apex Foot Products, South Hackensack, N.J.), Pelite (Durr-Filauer Medical, Inc., Chattanooga, Tenn.), PPT (Panger Biomechanics Group, Deer Park, N.Y.), and Sorbothane (Sorbothane, Inc., Kent, Ohio). Plastizote and Pelite are polyethylene foams, PPT is an open-cell polyurethane foam, and Sorbothane is a visco-elastic polymer.
In addition to single materials that are used to reduce force from high energy impacts, various composite materials where two or more components are intimately mixed together have been described. Examples of these composites include mixtures of fibers and ultrahigh molecular weight polyethylene (U.S. Pat. No. 4,946,721, issued Aug. 7, 1990); composites of rigid hollow bodies in an elastomeric matrix (U.S. Pat. No. 4,101,704, Issued Jul. 18, 1978) as well as various compositions of different elastomers and various fillers, for example, mixtures of rubber and latex as described by Portin in U.S. Pat. No. 4,082,888, issued Apr. 4, 1978.
Although such composites do offer some improvement in certain situations, often a structural composite composed of two or more layers of different materials offers additional protection not available by any one homogenous material alone. Such composite laminate structures have been specifically developed for many different applications. Some examples include shock absorbing athletic padding comprising a thermoplastic foam and a cellular rubber (U.S. Pat. No. 3,607,601, issued Sept. 21, 1971), an oriented foam having a thermoplastic film bonded to the surface (U.S. Pat. No. 3,619,344, issued Nov. 9, 1971), an impact absorbing laminate consisting of a layer of impact absorbing foam, a finishing layer, and a thin outer skin of substantially water impermeable resinous material (U.S. Pat. No. 3,816,234 issued Jun. 11, 1974), a protective device for the center of the chest comprising a stiff material that may have laminar cross sections (U.S. Pat. No. 5,245,706, issued Sep. 21, 1993), a resilient vehicular energy absorbing panel comprising a polyurethane foam core with a flexible reinforcing layer (U.S. Pat. No. 5,580,651, issued Dec 3, 1996) and many others. One laminate available commercially for the specific application of heel inserts described above is Spenco (Spenco Medical Corp., Waco, Tex.), which is a neoprene rubber foam with a nylon covering.
Applications that require materials or structures to reduce force from high energy impacts are different and distinct from the those used to reduce vibration. Vibration damping materials or systems are required where undesired resonances in a mechanical system may be excited by normal perturbations. The suspension system in an automobile, for example, will exhibit large unwanted oscillations in response to road irregularities unless properly damped. Shock absorbers, which produce forces opposing the velocity of compression or elongation of the springs, are employed to provide appropriate damping and inhibit oscillations. Such damping systems or materials are designed for periodic or recurring random changes of well defined loads, whereas impact energy absorbing materials such as those described herein are designed specifically for one time or, at most, infrequent impacts of high energy. Further, the goal in vibration damping is typically to reduce the maximum displacement after a perturbation, whereas impact energy absorbing materials reduce the transmitted force and minimize velocity changes resulting from an impact.
One material that apparently has not been examined as a high energy impact absorbing material is expanded polytetrafluoroethylene (abbreviated ePTFE) comprising polymeric nodes interconnected by fibrils defining a microporous structure. The processing and properties of this material are described by Gore in U.S. Pat. No. 3,953,566 (hereinafter referred to as '566) issued Apr. 26, 1976. Although polytetrafluoroethylene (abbreviated PTFE) (e.g., DuPont Teflon.RTM. fluoropolymer) has been described by Moschetti and Smith in U.S. Pat. No. 5,245,706, issued Sep. 21, 1993, as a material that could be used to protect against an impact in athletic wear, specifically a chest protector, they did not recognize the use of expanded PTFE. This material, which is available commercially in many forms, e.g., in rod form from W. L. Gore and Associates as Joint Sealant, has very different properties than granular Teflon.RTM. fluoropolymer materials, as fully described in '566. Because of its porous structure, ePTFE could inherently offer high energy impact energy absorption capability in much the same manner as porous polymeric foams or other materials, both being densified upon impact, thereby reducing the force transmitted through the material. Unlike many foams, though, the ePTFE may recover some or all of its ability to absorb another high energy impact because of its high strength and stiffness.
Similarly, other porous versions of PTFE may also offer improved impact energy resistance compared to dense, granular PTFE. Several different types of such materials have been prepared, primarily for use as an electrical insulation. Examples include the materials disclosed in U.S. Pat. No. 4,304,713 issued Dec. 8, 1981 to Perelman, and U.S. Pat. No. 4,663,095 issued May 5, 1987 to Battais. In the '713 patent, a volatile chemical blowing agent and a chemical foaming agent are employed with a perflurorocarbon resin to provide a foamed cellular structure. In the '095 patent, a mixture of PTFE, an aromatic pore-forming agent (e.g., benzene), a foaming agent, and a lubricating oil are reported to produce a foamed insulation. Finally, alternative methods of forming ePTFE have been described, for example, by McGregor, et. al. in U.S. Pat. No. 5,429,869 issued Jul. 4, 1995. In the '869 patent, PTFE and expandable thermoplastic microspheres are mixed and subsequently heated to form a coherent three dimensional expanded PTFE structure. In none of these cases was the use of such materials as a protection against high energy impacts disclosed.
Despite the developments described above, there continues to be a need for better materials and composites that function to mediate the effect of high energy impacts. Accordingly, it is a primary purpose of the present invention to produce a material that is capable of providing improved protection from high energy impacts. Specifically, a material that reduces the force generated from an impact as well as reduces the magnitude of the velocity change resulting from the impacts is desirable.
It is a further purpose of the present invention to provide a material with improved protection from high energy impacts, even after multiple impacts, instead of being essentially destroyed after the first impact like many traditional foamed polymers.
It is another purpose of the present invention to provide impact energy resistant materials that can be formed into multiple shapes, and therefore can be used in a wide variety of applications where protection is required.
It is another purpose of this invention to provide impact energy absorbing materials in forms that are well-suited to applications in constrained layers or other geometries that provide performance enhancements. This invention provides materials, which in film, sheet, rod, or other forms, may be laminated, pressure bonded, adhesively bonded, ultrasonically welded, or otherwise mechanically coupled, within structures such as constrained layers to yield maximal protection from high energy impacts. In addition, the invention yields materials with sufficient mechanical strength and integrity to provide good performance characteristics, including structural integrity, in laminates or other structures where shock absorbancy is required in conjunction with long term mechanical integrity.
It is yet another purpose of the present invention to provide impact energy resistant materials that are smooth and comfortable to the touch when placed against the human body.
These and other purposes of the present invention will become evident from review of the following specification.